Voltage source converters (VSC) are used for example in high voltage direct current (HVDC) systems, and also as Static Var Compensators (SVC). In the HVDC application, the voltage source converter is coupled between a direct current link and an alternating current network, and in the second application between a direct voltage source and an alternating current network. In both these applications, the voltage source converter must be able to generate an alternating current (AC) voltage of the same frequency as that of the alternating current network. The reactive and the active power flow through the converter is controlled by modulating the amplitude and the phase position, respectively, of the generated AC voltage in relation to the voltage of the alternating current network.
In particular the coming into being of voltage source converters equipped with series-connected transistors (IGBT) has made it possible to use this type of converters for comparatively high voltages, and pulse width modulation (PWM) for control of the generated AC voltage enables a very fast control of that voltage.
For a general description of controls systems for voltage source converters reference is made to Anders Lindberg: PWM and Control of Two and Three Level High Power Voltage Source Converters. Royal Institute of Technology, Department of Electric Power Engineering. Stockholm 1995, in particular pages 1, 21-56, 77-106, and appendix A, which are hereby incorporated by reference. This document is in the following referred to as Anders Lindberg for short.
FIG. 1 shows in the form of a schematic single line and block diagram a high voltage direct current transmission system as known in the prior art. A first and a second converter station STN1 and STN2 respectively, are coupled to each other via a direct current link having two pole conductors W1 and W2 respectively. Typically, the pole conductors are cables but they may also, at least to a part, be in the form of overhead lines. Although only the first station will be described in detail, it will be appreciated that the second station can be of the same design.
The converter station has capacitor equipment C1 coupled between the pole conductors, and comprises a voltage source converter (in the following VSC) VSC1. The VSC comprises semiconductor valves in a per se known bridge connection, such as, par example, a 2-level or a 3-level converter bridge as described in Anders Lindberg on pages 8-16. The semiconductor valves comprise, in a way known per se, branches of gate turn on/turn off semi-conductor elements, for example power transistors of so-called IGBT-type, and diodes in anti-parallel connection with these elements.
The voltage source converter is via a phase inductor PI1 and line L1 coupled to a three-phase alternating current electric power network N1. Although not shown in the figure, the phase inductor block PI1 also includes filter arrangements, such as a PLC/RI filter for filtering high frequency signals. As shown in the figure, it is well known in the art that the converter may be coupled to the three-phase network via transformers T1, in which case the phase inductors in some cases may be omitted.
When using a VSC comprising switchable semiconductors, such as IGBTs, the switching of the semiconductors introduces harmonic currents on the three-phase side of the VSC. These harmonic currents should preferably be filtered in the VSC station in order to avoid disturbances on the three-phase network N1. To this end, a first filter block F1 is coupled at a connection point between the phase inductor and the three-phase network. This first filter block is Y connected between the three phases and the neutral point is grounded. It will thus provide filtering of zero sequence currents appearing on the three-phase network.
A second filter block F2 is coupled at a connection point between the phase inductor and the three-phase network. This second filter block is ungrounded and Y connected between the three phases and since it is ungrounded, it will only provide filtering of plus and minus sequence currents.
The converter station comprises control equipment (not shown) for generation of trains of turn on/turn off orders to the semiconductor valves according to a predetermined pulse width modulation pattern. A control system for a voltage source converter in an HVDC transmission system is described in the European patent application with publication number EP 1 174 993.
It is desirable to minimize harmonic currents in the resistive part of filters, particularly in filters in very high voltage systems, in order to reduce losses and minimize power rating requirements on the resistors.